Question Communication satellites orbiting the moon

[Marcel]

When we finally get around to settling on the moon, we're going to need a network of communication satellites orbiting the moon. On Earth, we can use geosynchronous orbits. I assume that wouldn't work on the Moon, and we'll utilize lower orbits instead, but the thought popped into my head. Is a lunar synchronous orbit possible? How far out would it be at a period of one lunar month?

 

[Kaito]

If my understanding of the moon is correct, the satellite would be placed roughly at the center of the earth :rofl:

The same side of the moon always faces earth

 

[tblaxland]

Kaito is right.

For manned missions, we are unlikely to be settling on the far side of the Moon so Earth-based or Earth-orbiting relays could be used. For something like a farside unmanned radio telescope, a large amplitude L2 lissajous orbit would be the most likely.

 

[Nerull]

We haven't been smashing satellites into the moon for the fun of it (well, mostly...). Lunar orbit is unstable over long periods of time. Any satellite in lunar orbit needs lots of fuel to keep itself there, and has a limited lifespan. They would be better placed elsewhere - ground stations, Lagrange points, etc.

 

[Andy44]

Nerull is right about the instability of lunar orbits. The Moon's lumpiness means satellites have to do frequent maneuvers to stay in their orbits, or else they will either crash or be ejected.

Satellites may be cheaper in the short term, but once you get a permanent presence established on the lunar surface, your best option may be to place microwave relay towers on mountaintops in a great circle line from your farside base to the nearest place with constant Earth visibility.

The nice thing about there being no weather is that these towers will be very low maintenance, lasting for years without a visit by a repairman. Providing power would be more difficult, due to the long lunar nights, but there are ways to deal with that, too.

 

[Zatnikitelman]

If you really wanted satellites, you could place satellites in equatorial orbit around the moon and high enough so they have line of sight with each other. Depending on their altitude and how many you have, you could have continuous line-of-sight to all areas of the moon except some of the extreme polar areas.

 

[tblaxland]

Providing power would be more difficult, due to the long lunar nights, but there are ways to deal with that, too.

That's one reason for settling the poles, in preference to the mid latitudes. Get in the right spot on the pole and you will have continuous sunlight.

 

[Andy44]

Yes, and continuous Earth visibility, too, eliminating the need for comm sats or microwave towers.

But if you want to go anywhere on the farside it's a different story.

 

[Marcel]

I figured that a lunar synchronized orbit would be possible if the Moon was orbiting the Sun by itself, but the Earth would interfere with it in real life. I hadn't considered the "lumpiness" factor, but if the Moon was a perfect sphere, what would be the radius of a synchronous orbit? (I suck @ math)